In general, this invention relates to a cowling arrangement for a turbofan engine and, more particularly, to a variable cowling arrangement for a turbofan engine having variable pitch fan blades wherein an increased airflow can be admitted to the fan for improved performance efficiency during the reverse pitch mode of fan operation.
The new family of high bypass, large diameter, turbofan engines recently introduced into airline service have proved to be an economical and convenient means of propulsion for today's wide bodied aircraft which cruise subsonically. The high bypass engines have also demonstrated noise levels which are significantly lower than those of previous generation engines due to the reduced jet velocities of the airflow pressurized by the fan, while thrust levels have been increased as a direct result of the increased mass airflow through the fan. Even further increases in performance efficiency could be obtained if the fan speed could be varied to meet different engine operating requirements as dictated by the flight mission.
One means of varying the fan speed of a bypass turbofan engine without changing the rotational speed of the core engine is through the use of variable pitch fan blading. Such blading may be varied to cater to varying flight conditions and may be varied to reverse pitch for complete thrust reversal after the aircraft has landed. One difficulty which arises when the variable pitch fan blades are in reverse pitch for braking is that the airflow which is drawn into the fan from the rear may suffer some degree of distortion due to the turning of the airflow into the core engine compressor which still requires an inlet airflow for continued operation of the core engine. This distortion is due to the fact that the reverse pitch airflow passing forwardly must turn 180.degree. in order to enter the core engine inlet. As a direct consequence of such distorted flow, there is an aerodynamic restricting, or nozzle effect, produced in the airflow before it reaches the variable pitch fan blades. Since the entry to the fan is so restricted, the reverse thrust produced by the variable pitch fan is less than it would otherwise be if this aerodynamic restriction could be eliminated or reduced.
One proposed solution to the aforementioned problem has involved the use of a two part cowling around the variable pitch fan wherein the aft portion of the cowling has the ability to slide axially rearwardly with respect to the main forward portion, thus providing an annular intake to increase the reverse airflow to the fan. However, not only is it desirable that the aft portion of the cowling be made translatable to accommodate an increased airflow during reverse pitch, but there must also be means for varying the exit area from the cowling during normal operation at forward pitch. This is necessary in order to accommodate for the variations in mass airflow through the fan during the various modes of flight operation such as takeoff, climb to altitude, cruise and descent from altitude.
Therefore it is a primary object of this invention to provide a cowling arrangement for a turbofan engine having variable pitch fan blades wherein an increased airflow can be provided to the fan during reverse pitch operation while the exit area from the fan cowling can be varied during forward pitch operation to accommodate for the variations in mass airflow through the fan.
It is also an object of this invention to provide a cowling arrangement for a turbofan engine having variable pitch fan blades wherein the aft portion of the fan cowling has the ability both to slide axially with respect to the main forward portion providing an annular intake to increase reverse airflow, and to vary radially so as to adjust exit area during normal operation at forward pitch.